Chronic kidney disease inflicts a heavy toll on the individuals affected and on the health care resources of the nation. Few treatments are available to halt the inexorable progression of most kidney diseases. Recently, restrictions of dietary protein and phosphorous have been demonstrated to slow the progression of renal failure in several experimental models of kidney disease. Rigorously controlled studies in humans with kidney diseases are necessary to determine if similar dietary modifications are efficacious, nutritionally safe and tolerable. This study will determine if restrictions of dietary protein and phosphorous slow the rate of progression of renal insufficiency, lead to malnutrition or other adverse clinical conditions, and are acceptable to patients with renal insufficiency. As part of a multi-center, co-operative prospective randomized trial, patients with mild to moderate renal insufficiency due to progressive renal diseases will be assigned to follow their usual diet or a diet containing 0.7 g protein per kg body weight and 800 mg phosphorous per day. Thereafter both groups will be followed for three to five years to assess adherence to the diet, the rate of deterioation in glomerular filtration rate, and nutritional status. Patients will be cared for by their primary nephrologists and a nutritionist at New England Medical Center. The investigators are ideally suited to participate in this co-operative clinical trial because of their experience, commitment, and capabilities in performing clinical research and collaborating in cooperative studies. The Nephrology Division and the Frances Stern Nutrition Institute are widely known for the clinical services they provide and their previous and on-going clinical investigations. The investigators and key personnel have extensive referral networks and care for a large number of patients with progressive renal diseases. Based on a review of the current patient census, three are sufficient patients available for enrollment in the proposed trial. Moreover, the diet proposed was devised one year ago by members of the Nephrology Division and Frances Stern Nutrition Center and has already been prescribed for 32 patients. Although the patients were not part of a study, there were no apparent adverse consequences of the diet, and thus it is reasonable to select a similar diet for this study. Given their qualifications, access to patients and existing resources, the investigators are able to carry out all requirements to perform this important study.